drm/i915/bigjoiner: Avoid dsc_compute_config for uncompressed bigjoiner

For uncompressed big joiner DSC engine will not be used so will avoid
compute config of DSC.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Clinton Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514153711.2359617-15-matthew.d.roper@intel.com